Recently, in a display monitor, digital circuits are used in circuits for generating deflection correction waveforms for correcting display distortion and others, and a PLL circuit is used in a clock signal generating circuit used in a digital circuit. Conventionally, a horizontal drive pulse was generated in an analog horizontal synchronizing circuit of an exclusive integrated circuit (IC). However, when incorporating various functions of display monitor into a digital IC, generation of horizontal drive pulse is also accommodated in a same IC, and generation of horizontal drive pulse requires to use the PLL circuit for multiplying the clock signal used in the same digital circuit.
An example of such PLL circuit is shown in FIG. 4. In FIG. 4, when the frequency dividing ratio of a dividing circuit 4 is set at a specified value N, and the PLL is in locked state, and output signal of a VCO (voltage controlled oscillator) 3 is synchronized with a reference signal entered in a phase comparator 1, and its oscillation frequency is N times of the frequency of the reference signal. In the PLL circuit, the frequency dividing ratio of the dividing circuit 4 may be sometimes fixed, but it may be also controlled by a CPU 5 depending on the frequency of the entered reference signal as shown in the diagram.
In the PLL circuit, in order to decrease jitter generated in the reproduced image shown in the display monitor due to phase fluctuation or phase noise of the PLL circuit, it is preferred to use an oscillator of an excellent frequency stability in the VCO. When an oscillator of an excellent frequency stability is used in the VCO, the control sensitivity of the oscillation frequency of the VCO is often lowered. Generally, since the voltage width to be controlled is limited, the variable frequency range capable of controlling the oscillation frequency of the VCO is narrow. For example, the resonance type VCO high in frequency stability is narrower in the controllable variable frequency range as compared with the VCO of multi-vibrator type or ring oscillator type.
However, due to variations of constants of the parts being used, the oscillation frequency oscillated by the VCO varies, and the VCO often oscillates at a frequency exceeding an expected frequency range. Therefore, when designing the circuit, it is necessary to determine the stability of oscillation frequency in consideration of variation of constituent parts used in the circuit. By such designing, to the contrary, the stability of oscillation frequency of VCO is lowered and the jitter performance declines.
Accordingly, to suppress variations of oscillation frequency, the circuit designers attempted to limit the VCO composing parts to the parts particularly small in allowable deviation, and adjust individually the oscillation frequency in the production process by using variable capacity capacitors or the like.
As a result, the cost of the appliances using such circuits was raised, and the productivity was impaired.